1. Field of the Invention
The present invention relates to the field of thin film formation, and more particularly to a method and apparatus for depositing an insitu boron doped amorphous or polycrystalline silicon film.
2. Discussion of Related Art
Polycrystalline silicon (polysilicon) and amorphous silicon thin films are used throughout the many semiconductor integrated circuit manufacturing processes. These films are used, for example, in the fabrication of gate electrodes, stack or trench capacitors, emitters, contacts, fuses, and antifuses. As device dimensions decrease to below 0.25 microns in order to increase packing density, aspect ratios (aspect ratio=depth/width) of holes, vias, and trenches in the integrated circuit are also increasing. In order to fill high aspect ratio openings (aspect ratiosxe2x89xa7to 2.5), deposition processes which are capable of good step coverage (step coverage %=film thickness on a step surface/film thickness on a flat surfacexc3x97100%) are required to ensure complete hole filling without the creation of voids.
One current method which can provide adequate step coverage is low pressure chemical vapor deposition (LPCVD). In LPCVD processes, reaction vessels are evacuated to relatively low pressures of between 100-1000 m torr. The low pressures associated with LPCVD processes cause silicon films to be deposited at low rates (about 100 angstroms (xc3x85)/minute for undoped films and about 20 xc3x85/minute for doped films). The low deposition rates enable the films to be deposited with good step coverage. When n-type dopants are introduced in a LPCVD batch system to produce an insitu doped film, step coverage decreases. A further reduction in the deposition rate is necessary for good step coverage. Although LPCVD processes can form high quality films, their low deposition rates necessitate the processing of multiple wafers (i.e. up to 100) at one time in a batch type reaction vessel. A problem with processing a plurality of wafers in a single machine at a single time is that it is difficult to obtain uniform thickness film and dopant concentration from wafer to wafer and from batch to batch.
To fabricate polysilicon and amorphous silicon films with precise thickness and doping uniformity across a wafer and from wafer to wafer, single wafer CVD processes are used. A single wafer CVD process for producing a silicon layer on a silicon wafer is described in U.S. Ser. No. 07/742,954, filed Aug. 9, 1991, entitled Low Temperature High Pressure Silicon Deposition Method and is assigned to the present assignee. Such a single wafer reactor can reliably form a uniform silicon film which is insitu doped with n type dopants (e.g. arsenic and phosphorus).
At times, however, such as in the manufacture of Flash memory devices and p channel devices, it is desireable to form amorphous or polycrystalline films which are insitu doped with p type dopants (e.g. boron).
Thus, what is desired is a method and apparatus which enables an insitu boron doped polycrystalline or amorphous silicon film to be deposited in a single wafer reactor without forming deposits on the chamber windows and liners.
A method and apparatus for depositing an insitu boron doped amorphous or polycrystalline silicon film on a substrate. According to the present invention, a substrate is placed into a deposition chamber. A reactant gas mix comprising a silicon source gas, a boron source gas, and a carrier gas are fed into the deposition chamber. The reactant gas mix is fed into the deposition chamber at a rate so that the residence time of the reactant gas in the deposition chamber is less then or equal to 3 seconds or alternatively has a velocity of at least 4 inches/sec.
In another embodiment of the present invention an insitu boron doped amorphous and polycrystalline silicon film, a substrate is placed into a deposition chamber. The substrate is then heated to a deposition temperature between 580-750xc2x0 C. and the chamber pressure reduced to less than or equal to 50 torr. A silicon source gas is then fed into the deposition at a rate to provide a silicon source gas partial pressure of between 1-5 torr while a boron source gas is fed into the deposition chamber at a rate to provide a boron source gas partial pressure of between 0.005-0.05 torr.